San Lorenzo is among the oldest churches in Florence, the city's cathedral before Santa Maria del Fiore. In 1400, it was made bigger, at the behest of the wealthy banker Giovanni dei Bicci, head of the Medici family, who lived in the neighbourhood and wanted to make it the family's church of reference. The architect who was already working on his chapel, the Old Sacristy, was Filippo Brunelleschi. When Giovanni died, the works continued under the tutelage of his son Cosimo the Elder, and only after 1441 did the direction of the works pass to Michelozzo, Brunelleschi's successor and architect of the Medici-Riccardi Palace. With the death of Cosimo the Elder and his burial in the underground crypt, San Lorenzo became the burial place for the members of the Medici family, a tradition that continued until the family died out.
The facade of the church remained unfinished, despite the fact that when the son of Lorenzo the Magnificent was elected Pope Leo X, Michelangelo was commissioned to find the best marble extracted from the Medici quarries in Serravezza. Leo X also commissioned the New Sacristy, in order to preserve the tombs of the two young men of the Medici family, Lorenzo Duke of Urbino and Giuliano Duke of Nemours, both of whom died in the age of thirty. The work was also carried out to include the tombs of Lorenzo the Magnificent and his brother Giuliano.Inside the Basilica, there is the room where Michelangelo stayed for a few months. The artist took refuge there in 1530, to escape the revenge of the Medici family, in exile after his betrayal in favour of the rebels of the popular revolt that had ousted them. During his stay, he made numerous charcoal studies that remained on the walls for centuries. A real and still unsolved mystery lies in San Lorenzo: the starry sky in the intrados of the internal dome of the Old Sacristy is a portrayal of the night sky over Florence. Giuliano d'Arrigo, known as Pesello, painted the perfect location of the stars on the night of 4 July 1442, with astronomical patterns represented by the zodiac signs studied by the astronomer Paolo dal Pozzo Toscanelli. The precision of the astronomical observation of the starry sky was verified by Professor Forti, director of the Astronomical Observatory of Arcetri in the 1980s. Even today, scholars do not know how to give an unequivocal explanation for the choice of this precise date portrayed in San Lorenzo, just like the identical one created in the Pazzi Chapel in Santa Croce.
Clement VII, Pope from the Medici family, commissioned Michelangelo to enrich San Lorenzo by creating the marvellous Laurentian Library, a place to keep the most important manuscripts in the world. Ferdinando I initiated the Cappella dei Principi, behind the main altar, which was one of the Medici family's last great enterprises.
The interior of the church is decorated with valuable works, one of which is the Marriage of the Virgin by Rosso Fiorentino.